1. Field of the Invention
This invention relates to devices for establishing electrical contact to an integrated circuit (IC) chip, and more particularly to a test socket for high speed surface mount IC chips whose testing is sensitive to added capacitances and inductances.
2. Description of the Related Art
Surface mounting, in which the leads of an IC chip are soldered directly to a circuit board, has become a popular packaging technique. Various lead configurations have been developed, such as J-type, gull-winged, butt-leaded and leadless packages, in which leads or contact pads on the chip are soldered directly to the circuit board. Surface mount packaging is discussed in general and illustrated in Sze, ed., VLSI Technology, 2d Ed., McGraw-Hill Book Company, 1988, pages 569-570.
A number of different test sockets have been developed that allow surface mount chips to be temporarily connected to a test board without being soldered in place, and then removed from the socket when the testing has been completed. The ideal test socket would be inexpensive, have a long life time, not add significant amounts of capacitance or inductance that can introduce errors when testing high speed chips, and be adaptable to different chip configurations. While numerous test sockets are available that provide an electrical interface between a chip to be tested and a test board, none of them satisfy all of the above requirements.
One test socket, produced by the Cinch division of Labinal Components and Systems, Inc. under the trademark CIN.APSE, uses gold plated wire that is rolled into springy masses and captured in buttons to provide electrical connections between the leads of a device under test (DUT) and contact pads on a test board. Pressing the chip down against the array of buttons establishes electrical contacts between the chip pins and the test board pads on opposite sides of the buttons. This type of device is described in the brochure "CIN.APSE A New Patented Technology Bringing PC Interconnects Up To Speed" by Labinal Components and Systems, Inc., 1989. While the test socket is capable of establishing firm electrical connections between the chip and the test board, it is quite expensive and has a short life time, on the order of 20,000 chip insertions, which typically represents approximately three weeks of use.
Another structure that has been used for testing surface mount chips is an elastomeric connector produced under the trademark SILVER STAX by PCK Elastomerics, Inc. The connectors consist of conductive silver impregnated layers of silicone that are separated by nonconductive silicone layers. The dimensions of the lamination can be selected so that successive conductive layers align with successive leads of a DUT, with a separate clamp used to hold the DUT in place. The connectors are described in a technical data sheet entitled "SILVER STAX Elastomeric Connectors" by Elastomeric Technologies, Inc., February 1989. When used for surface mount chip testing, the connectors are normally inserted into slots within a test socket housing. In volume testing applications the elastomeric connectors generally have an even shorter life time than the CIN.APSE test socket, typically not more than one week. Furthermore, they are subject to a buildup of oxide on the silvered conductors, which can lead to intermittent contacts.